diff --git a/lib/delayed/backend/mongo_mapper.rb b/lib/delayed/backend/mongo_mapper.rb
index cafad14..9757373 100755
--- a/lib/delayed/backend/mongo_mapper.rb
+++ b/lib/delayed/backend/mongo_mapper.rb
@@ -68,7 +68,9 @@ def self.reserve(worker, max_run_time = Worker.max_run_time)
 
             # Return result as a MongoMapper document.
             # When MongoMapper starts supporting findAndModify, this extra step should no longer be necessary.
-            self.first :conditions => {:_id => result["_id"]}
+            if result
+              self.first :conditions => {:_id => result["_id"]}
+            end
           rescue Mongo::OperationFailure
             nil # no jobs available
           end